Egg whites are mostly protien. Well, mostly water, but the water doesn't count. It's used for structure, volume and leavening depending what's done with them. They have little flavor.
The yolk has the fat, emulsifiers and other interesting things including some other protiens It's used to bind ingredients that wouldn't otherwise mix as in mayonaisse, add color, richness and some flavor. It can also thicken things when partially cooked as in custards, hollandaise and such.
Eldon, here is a perfect example of why you should get the book "On food and cooking" by Harold McGee it will answer that question in a whole lot more depth. Not to say that we are chopped liver :cool:
Go to the "special Guest Archive" section and read the posts from when Harold visited us.